Communication systems that geographically reuse communication resources are known. These systems allocate a predetermined set of communication resources in one geographic area and reuse the same set of communication resources in one or more other geographic areas. This reuse technique improves communication capacity by minimizing the number of communication resources necessary to provide communication service in a large geographic area comprised of several smaller geographic areas.
As is also known, a communication resource is defined by the multiplexing scheme utilized in the communication system. For example, with frequency division multiplexing (FDM), the communication resource may be a frequency carrier or pair of frequency carriers; whereas,with time division multiplexing (TDM), the communication resource may be a time slot or pair of time slots in one or more time frames. Further, an assigned code pattern may provide the communication resource in a code division multiplexing (CDM) system.
The most common communication system which geographically reuses communication resources is a cellular system. In a cellular system, allocation of a communication resource begins when a communication unit requests communication service. Based on resource availability and signal usability, a resource controller assigns the communication resource to the communication unit. A communication, such as a conversation or a facsimile transmission, occurs on the communication resource between the communication unit and another communication unit or between the communication unit and a subscriber to a public service telephone network. The communication continues until completion or an interruption in service occurs. In the event of a potential interruption in service, the communication may be reassigned, or handed off, to an alternate communication resource for continuation. The alternate communication resource is generally selected based on the same qualities used by the resource controller to allocate the original communication resource. Upon conclusion of the communication, the resource controller retrieves the communication resource; thus making the communication resource available for another communication.
An important parameter in identifying an acceptable communication resource is signal usability. In a wireless communication system, the communication resources are typically radio frequency (RF) channels that occupy predetermined bandwidths or time slots in predetermined time frames. When information signals are transmitted on the communication resources, undesired effects, such as fading and interference, alter the information signals during transmission. Thus, the information signals received by a receiver in the communication unit, or a broadcast unit, are corrupted by the undesired effects. By ascertaining an indication of the corruption on available communication resources, the least corrupted communication resource may be selected for the communication. This indication of corruption is known as signal usability.
Received signal strength indication (RSSI) and bit error rate (BER) are common methods of estimating signal usability. In an RSSI estimate, the receiver measures a summation of signal levels including the desired information signal, co-channel interference, and noise on the desired RF channel. Although this technique accurately estimates the level of the received signal, it cannot distinguish between the desired information signal and signals due to co-channel interference. Thus, an acceptable RSSI measurement may not indicate acceptable signal usability due to a high level of co-channel interference. Alternatively, BER measurements provide accurate estimates of signal usability, but in geographic areas where error rates are low, multiple measurements and excessive averaging times may be required to obtain the accurate estimates. Thus, measurement periods of ten to fifty seconds may be necessary to obtain accurate BER data.
When the RSSI or BER indicates an unusable communication resource, the communication is handed off from the unusable communication resource to another communication resource. As is known, both the communication unit and the broadcast unit involved in the communication may monitor the usability of the active communication resource and initiate the handoff process upon degradation to it. However, the usability of the active communication resource is not monitored by both units simultaneously. After a handoff is requested, either the broadcast unit, the communication unit, or both determine the alternate communication resource with which to reassign the communication. Generally, the communication unit monitors the active resource's outbound link (from broadcast unit to communication unit) and the broadcast unit monitors its inbound link (from communication unit to broadcast unit). Therefore, when the communication unit requests a handoff, the outbound link has degraded signal usability and when the broadcast unit requests a handoff, the inbound link has degraded signal usability.
Upon initiation of the handoff process, selection of a new communication resource on which to continue the communication is typically approached in two ways. After a handoff is requested due to a degraded inbound link, scanning receivers in broadcast units associated with alternate communication resources measure corresponding inbound link RSSI or BER for the communication. The broadcast units report their measurements to the central controller which selects the communication resource with the best inbound link measurement. This approach ignores the usability of the selected resource's outbound link and utilizes unpreferred methods for determining signal usability. Similarly, after a handoff is requested due to a degraded outbound link, the communication unit obtains RSSI or BER measurements of alternate outbound links and the communication resource with the best outbound link measurement is selected. This approach ignores the usability of the selected resource's inbound link. Further, both approaches do not limit the number of alternate communication resources being considered as handoff candidates, which can be quite large. Thus, additional handoff delays and service interruptions may result.
Therefore, a need exists for a rapid, anticipatory method for determining when to hand off a communication from one communication resource to another based on signal usability of inbound and outbound links.